My immediate practical goal is to be able to select members of two different groups. The query should be something like ‘SELECT uid FROM group_member WHERE gid=my_gid AND uid in (SELECT uid FROM group_member WHERE gid=my_other_gid)’ – for example to cross special interest groups or geographical areas.

There is plenty of potential for useful data mining that is not exposed by Facebook’s default interface. Search with multiple criteria of the same category is an obvious need for finding interesting people. Maybe did Facebook decide that the cost of additional clutter was not worth it for the average user. Or maybe they would prefer that the users don’t realize how much information can emerge from mining their data…